A flow is a more or less directed movement of particles. In case of a desired flow this property is utilized in order to transport the particles over a distance with their directed movement. A flow may also be an unwanted side effect caused for instance by the fact that objects move relative to a medium.
For instance, in aviation it has been known for a long time that planes generate, as soon as their nose wheel is lifted from the ground, at the end of their wing tips air turbulence which is commonly known as wake turbulence. Wake turbulence is a vortical air flow behind a flying aircraft which consists of two contra-rotating air turbulences. Between these two vortex cores—which usually have a distance of three quarters of the wingspan of the aircraft—there are strong vertically descending air currents, while outside of the vortex cores there are strong upcurrents which can be very dangerous for successional aircrafts. An aircraft, which is, for instance, with one half of its wing between the downwash of the vortex cores and with its other half of the wing in the area of the upcurrent will get into a dangerous roll. Thus, for some aircraft accidents, the cause was seen in the fact that a following aircraft had gotten into the wake turbulence of a preceding aircraft and due to the strong vertical air movements finally crashed which in practice is the most common cause of incidents prior to landing.
This phenomenon of wake turbulence occurs especially during the landing of a large aircraft, thus, for safety reasons, after the landing of an aircraft it is required to wait a while until the same runway can be used again for air traffic. This safety measure is to ensure that aircrafts do not get into wake turbulence of a previously landed aircraft during take off or landing, and thus eventually crash. The effect duration of such wake turbulence depends on external environmental factors such as temperature, air pressure and wind direction and wind velocity. Until now the existence of such a wake turbulence can not be detected reliably; therefore a sufficient waiting time period (up to five minutes) is used for safety reasons after the landing of an aircraft till flight traffic is permitted again on this runway, regardless of whether the wake turbulence still exists or not. This does have a negative effect on the ultimate utilization of the airport.
In addition to the landing phase wake turbulences are also a problem at cruising levels. Due to the dense traffic resolution in air space it happens more and more frequently that a flying aircraft is getting into the trail of a preceding or higher flying aircraft (wake turbulences usually drop). Strong accelerations on board are usually the result which can result in injury to persons on board who are not wearing seat belts.
From a state of the art point of view a number of measures are known to avoid such wake turbulences. For instance, so-called winglets are used to reduce the turbulence of the air flow at the end of the wings. Winglets are mounted extensions at the end of the wings of aircrafts which are usually arranged basically vertical to the wing. For example, from DE 10 2007 059 455 A1, a wing tip extension is known which is to moderate the emergence of wake turbulence of such an aircraft.